"On what day does the Sun do it's bounce along the horizon and viewed from where ?"
You'll have to wait till the book comes, it will probably be 21st June, not always convinced by Devereux, but Pete put a photo up not so long ago, of a view from the top of Silbury, and it must have been that horizon towards Marlborough I think... As for Picked Hill, its a strange old hill that maybe could be seen from Marden, and of course it lies in the corridor of the Vale of Pewsey between Avebury/Stonehenge.....